Neurodegenerative diseases such as Alzheimer and Parkinson""s diseases share a common pathological characteristic, i.e., the deterioration of nerve cell connections within the nervous system. As a consequence of the disruption of normal neuronal connections, patients suffer from a number of cognitive deficits including impaired learning and memory.
The central element of neuronal networks is the xe2x80x9csynapsexe2x80x9d, which denotes the structural specialization of the junctional contact between two nerve cells. A synapse is a highly sophisticated electrochemical device composed of a presynaptic terminal and a specialized postsynaptic membrane. Only by establishing synaptic connections, can nerve cells organize into networks and acquire information processing capability such as learning and memory. Synapses are progressively reduced in number during normal aging, and are severely disrupted during neurodegenerative diseases, Alford, M. E. et al., J. Histochem. and Cytochem, 42:283-7 (1994), Lassmann, H. et al., Ann. NY Acad. Sci., 695:59-64 (1993), Zhan, S. S. et al, Acta Neuropathologica 86:259-264 (1993). Clinical dementia, the most common symptom of neurodegenerative diseases is best corrected with the severity of synaptic deterioration in the central nervous system, Samuel W. et al., Archives of Neurology, 51:772-8 (1994), Masliah, E et al., Medical Hypothesis, 41: 334-340 (1993), Zhan, S. S. et al., Dementia, 5: 79-97, (1994). Therefore, finding molecules capable of creating and/or maintaining synaptic connections is an important step in the treatment of neurodegenerative diseases.
During recent years, a great deal of effort has been made by investigators to characterize the function of synaptic proteins, i.e., proteins enriched in synapses. Examples of synaptic proteins with recently characterized functions are numerous, and include synaptotagmin, syntexin, synaptophysin, synaptobrevin, and the synapsins. In contrast to other synaptic proteins which are known to be involved in specific aspects of synaptic function, e.g., synaptic vesicle recycling or docking, the synapsins are now known to play a much broader organizational role in axonogenesis, in the differentiation of presynaptic terminals, and in the formation and maintenance of synaptic connections.
Synapsin I and synapsin II are a family of neuron-specific phosphoproteins which are highly concentrated in adult nerve terminals. Synapsin I and synapsin II are encoded by two genes, the synapsin I gene and the synapsin II gene. Alternative splicing of the primary transcripts of synapsins I and II genes gives rise to their protein products synapsins Ia and Ib and synapsin Iia and Iib which are collectively termed the synapsins. The four members of the synapsin family (synapsin Ia, Ib, IIa and IIb) share a high degree of homology in their cDNA and amino acid sequences. Domains A, B, C, are highly conserved common domains of the synapsin family and together occupy more than 80% of synapsin IIb, the shortest isoform of the family. Both synapsin I and II have been cloned and sequenced, Greengard et al., Science 259:780-785 (1993).
In mammals, the ontogeny of the synapsins coincides with the terminal differentiation of neurons, and the levels of expression of the synapsins parallel the formation of synapses in the nervous system. The synapsins exhibit a distinct pattern of distribution, being expressed only in the nervous system, present only in neurons but not in glial cells, and specifically localized in the presynaptic compartment of the synapses in adult nervous system where they are associated with the cytoplasmic surface of synaptic vesicles. In vitro binding analysis indicates that synapsins are able to interact with actin and other cytoskeletal elements in a phosphorylation dependent manner. Both synapsin I and synapsin II are able to bundle filamentous actin, and phosphorylation of synapsin by protein kinases leads to a reduction in actin-bundling capability. Transfection of synapsins, regardless of isoform, into fibroblast cells resulted in a remarkable reorganization of cytoskeleton and the formation of highly elongated cellular processes, Han and Greengard, PNAS, 91:8557-8561 (1994). Synapsins are also able to interact with synaptic vesicles in a phosphorylation-sensitive fashion. Both synapsin I and synapsin II are able to bind to the cytoplasmic surface, and the binding affinity of synapsin to synaptic vesicles is regulated by phosphorylation. Thus, the synapsins are capable of interacting with multiple macromolecular components within the nerve terminal. Currently, the effects of synapsins on the organization of actin cytoskeleton are thought to be a cell biological basis underlying synapsin""s function in neuronal development, De Camilli, P. et al., Annu. Rev. Cell Biol. 6:433-460, (1990), Valtorta et al., J. Biol. Chem, 267:7195-7198 (1992) and Greengard et al., Science 259:780-785 (1993).
Synapsin I and synapsin II have been intensively analyzed for their role in the regulation of neurotransmitter release from adult nerve terminals. A large body of experimental evidence shows that the synapsins are important regulatory molecules that control synaptic release of neurotransmitters, Greengard et al., Science 259:780-785 (1993).
The first demonstration of synapsins effect on neuronal cell development came from a transfection experiment in which cDNA encoding synapsin IIb was introduced to a cell line NG108-15, Han, et al., Nature, 349:697-700 (1991). NG108-15 is a line of hybrid cells made by cell fusion between mouse neuroblastoma and rat glioma cells. When treated with agents that raise the intracellular cyclic AMP level, this cell line undergoes differentiation and becomes neuronal-like. When synapsin IIb was overexpressed by transfection, NG108-15 cells unexpectedly acquired a much stronger neuronal phenotype: having more neuritic varicosities (nerve terminals) per cell, more synaptic vesicles per varicosity, and more synaptic vesicle-associated proteins. Thus, synapsin IIb and possibly other synapsins (based on their high sequence homologies) are implicated in the formation of presynaptic terminals.
Subsequent studies performed in a totally different system, i.e., the frog embryos, provided further supportive evidence for the role of synapsins in nerve cell development. Injection of synapsin protein into early developing frog embryo (at several cell stage) caused the nerve cells (which came into being 24 hours after the injection into form synapses with muscle cells more effectively, Lu et al., Neuron 8:521-529 (1992).
These results suggested that the synapsins may play a role in synaptogenesis. However, the experimental approaches used in the above experiments were insufficient in establishing a clear relationship between synapsins and synaptogenesis due to the fact that the systems used did not involve a pure neuronal context. The NG108-15 cells are not real neurons and the results obtained need further verifications using real neurons. In the frog embryo experiment, synapsin was not directly injected into developing neuronal cells but rather into a several cell-stage embryo. Therefore there was a lack of direct evidence for the effects of synapsins obtained from a pure neuronal system.
The present invention relates to the discovery of the role of synapsin II in a pure neuronal system, and the concomitant utilities available for therapy from these discoveries.
It is an object of the present invention to provide a method of maintaining and/or restoring synapses in a patient in need of therapy for a neurodegenerative disorder by administration of an agent in an amount sufficient to maintain and/or restore synapses.
It is a further object of the present invention to provide a method of treatment for neurodegenerative disorders which comprises administration to a patient in need of such treatment an amount sufficient to maintain and/or restore synapses of a therapeutic agent capable of maintaining and/or restoring synapses.
It is a still further object of this invention to provide a method of maintaining and/or restoring synapses by the administration of the synapsin cDNAs or proteins into the patient""s nervous system.
It is an object of the present invention to provide a method of maintaining and/or restoring synapses by administration of the synapsin cDNAS to promote the synapse forming ability of cells for grafting.
It is a further object of this invention to provide a method of maintaining and/or restoring synapses by the administration of an agent that increases the expression of, or enhances the activity of, the endogenous synapsins.
It is a still further object of the present invention to provide a method of treatment for Alzheimer disease by administration to a patient in need of such treatment an amount sufficient to maintain synapses of a therapeutic agent which mimics the activity of synapsin and is thus capable of maintaining and/or restoring synapses.
The present invention relates to a method of maintaining and/or restoring synapses in a patient in need of therapy for a neurodegenerative disorder by administration of an agent in an amount sufficient to maintain and/or restore synapses. More particularly, the present invention concerns a method of treatment for neurodegenerative disorders which comprises administration to a patient in need of such treatment an amount sufficient to maintain and/or restore synapses of a therapeutic agent capable of maintaining and/or restoring synapses.